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A variety of proteins can be encoded by a single gene via the differential splicing of exons. In neurons this form of alternative splicing can be controlled by activitydependent calcium signaling, leading to the properties of proteins being altered, including ion channels, neurotransmitter receptors and synaptic cell adhesion molecules. The pre-synaptic cell adhesion molecule Neurexin 1 (Nrxn1) is alternatively spliced at splice-site 4 (SS4) which governs exon 22 inclusion (SS4+) and consequently postsynaptic NMDA receptor responses. Nrxn1 was reported to be subject to a delayed-onset shift in Nrxn1 SS4 splicing resulting in increased exon 22 inclusion, involving epigenetic mechanisms which, if disrupted, reduce memory stability. Exon inclusion at SS4 represented one of hundreds of exons reported to be subject to a genome-wide shift in fractional exon inclusion following membrane depolarization with high extracellular K+ that was delayed in onset. We report that high K+ does not increase the SS4+/SS4 ratio in cortical neurons, but does induce a delayed-onset NMDA receptor-dependent neuronal death. In mixed neuronal/astrocyte cultures this neuronal death results in an increase in the astrocyte: neuron ratio, and a misleading increase in SS4+/SS4 ratio attributable to astrocytes having a far higher SS4+/SS4 ratio than neurons, rather than any change in the neurons themselves. We reassessed the previously reported genome-wide delayed-onset shift in fractional exon inclusion after high K+ exposure. This revealed that the reported changes correlated strongly with differences in exon inclusion level between astrocytes and neurons, and was accompanied by a strong decrease in the ratio of neuron-specific: astrocytespecific gene expression. As such, these changes can be explained by the neurotoxic nature of the stimulation paradigm, underlining the importance of NMDA receptor blockade when using high K+ depolarizing stimuli. [ABSTRACT FROM AUTHOR]

Background: Mutations in the postsynaptic transmembrane protein neuroligin-3 are highly correlative with autism spectrum disorders (ASDs) and intellectual disabilities (IDs). Fear learning is well studied in models of these disorders, however differences in fear response behaviours are often overlooked. We aim to examine fear behaviour and its cellular underpinnings in a rat model of ASD/ID lacking Nlgn3. Methods: This study uses a range of behavioural tests to understand differences in fear response behaviour in Nlgn3−/y rats. Following this, we examined the physiological underpinnings of this in neurons of the periaqueductal grey (PAG), a midbrain area involved in flight-or-freeze responses. We used whole-cell patch-clamp recordings from ex vivo PAG slices, in addition to in vivo local-field potential recordings and electrical stimulation of the PAG in wildtype and Nlgn3−/y rats. We analysed behavioural data with two- and three-way ANOVAS and electrophysiological data with generalised linear mixed modelling (GLMM). Results: We observed that, unlike the wildtype, Nlgn3−/y rats are more likely to response with flight rather than freezing in threatening situations. Electrophysiological findings were in agreement with these behavioural outcomes. We found in ex vivo slices from Nlgn3−/y rats that neurons in dorsal PAG (dPAG) showed intrinsic hyperexcitability compared to wildtype. Similarly, stimulating dPAG in vivo revealed that lower magnitudes sufficed to evoke flight behaviour in Nlgn3−/y than wildtype rats, indicating the functional impact of the increased cellular excitability. Limitations: Our findings do not examine what specific cell type in the PAG is likely responsible for these phenotypes. Furthermore, we have focussed on phenotypes in young adult animals, whilst the human condition associated with NLGN3 mutations appears during the first few years of life. Conclusions: We describe altered fear responses in Nlgn3−/y rats and provide evidence that this is the result of a circuit bias that predisposes flight over freeze responses. Additionally, we demonstrate the first link between PAG dysfunction and ASD/ID. This study provides new insight into potential pathophysiologies leading to anxiety disorders and changes to fear responses in individuals with ASD. [ABSTRACT FROM AUTHOR]

The human brain generally remains structurally and functionally sound for many decades, despite the post-mitotic and non-regenerative nature of neurons. This is testament to the brain’s profound capacity for homeostasis: both neurons and glia have in-built mechanisms that enable them to mount adaptive or protective responses to potentially challenging situations, ensuring that cellular viability and functionality is maintained. The high and variable metabolic and mitochondrial activity of neurons places several demands on the brain, including the task of neutralizing the associated reactive oxygen species (ROS) produced, to limit the accumulation of oxidative damage. Astrocytes play a key role in providing antioxidant support to nearby neurons, and redox regulation of the astrocytic Nrf2 pathway represents a powerful homeostatic regulator of the large cohort of Nrf2-regulated antioxidant genes that they express. In contrast, the Nrf2 pathway is weak in neurons, robbing them of this particular homeostatic device. However, many neuronal antioxidant genes are controlled by synaptic activity, enabling activity-dependent increases in ROS production to be offset by enhanced antioxidant capacity of both glutathione and thioredoxin-peroxiredoxin systems. These distinct homeostatic mechanisms in neurons and astrocytes together combine to promote neuronal resistance to oxidative insults. Future investigations into signaling between distinct cell types within the neuro-glial unit are likely to uncover further mechanisms underlying redox homeostasis in the brain. [ABSTRACT FROM AUTHOR]

Microglia, brain-resident macrophages, require instruction from the CNS microenvironment to maintain their identity and morphology and regulate inflammatory responses, although what mediates this is unclear. Here, we show that neurons and astrocytes cooperate to promote microglial ramification, induce expression of microglial signature genes ordinarily lost in vitro and in age and disease in vivo , and repress infection- and injury-associated gene sets. The influence of neurons and astrocytes separately on microglia is weak, indicative of synergies between these cell types, which exert their effects via a mechanism involving transforming growth factor β2 (TGF-β2) signaling. Neurons and astrocytes also combine to provide immunomodulatory cues, repressing primed microglial responses to weak inflammatory stimuli (without affecting maximal responses) and consequently limiting the feedback effects of inflammation on the neurons and astrocytes themselves. These findings explain why microglia isolated ex vivo undergo de-differentiation and inflammatory deregulation and point to how disease- and age-associated changes may be counteracted. [Display omitted] • Neurons and astrocytes combine to promote the microglial homeostatic signature • This rescues changes that happen in microglia ex vivo and in disease • Mechanistically, this involves transforming growth factor beta 2 (TBGF-β2) signaling • Neurons and astrocytes also repress microglial inflammatory responses Baxter et al. show that the transformation of microglia from a healthy to disease-associated state can be suppressed by the combined actions of neurons and astrocytes via a mechanism involving TBGF-β2 signaling. They also repress exaggerated microglial responses to mild stimuli and limit the feedback signaling from activated microglia. [ABSTRACT FROM AUTHOR]

Summary The GluN2 subtype (2A versus 2B) determines biophysical properties and signaling of forebrain NMDA receptors (NMDARs). During development, GluN2A becomes incorporated into previously GluN2B-dominated NMDARs. This "switch" is proposed to be driven by distinct features of GluN2 cytoplasmic C-terminal domains (CTDs), including a unique CaMKII interaction site in GluN2B that drives removal from the synapse. However, these models remain untested in the context of endogenous NMDARs. We show that, although mutating the endogenous GluN2B CaMKII site has secondary effects on GluN2B CTD phosphorylation, the developmental changes in NMDAR composition occur normally and measures of plasticity and synaptogenesis are unaffected. Moreover, the switch proceeds normally in mice that have the GluN2A CTD replaced by that of GluN2B and commences without an observable decline in GluN2B levels but is impaired by GluN2A haploinsufficiency. Thus, GluN2A expression levels, and not GluN2 subtype-specific CTD-driven events, are the overriding factor in the developmental switch in NMDAR composition. Graphical Abstract Highlights • Mutating the GluN2B CaMKII site affects phosphorylation of its C-terminal domain • The developmental changes in NMDAR composition and synaptogenesis occur normally • Changes in NMDAR composition do not require distinct GluN2 C-terminal domains • Developmental changes in NMDAR composition are primarily sensitive to GluN2A levels An important milestone in forebrain neuronal development is the switch in composition of the NMDA receptor: GluN2A becomes incorporated into previously GluN2B-dominated NMDARs. Using knockin mice, McKay et al. find that, contrary to earlier proposed models, the switch does not require GluN2A and GluN2B to possess distinct C-terminal domain sequences. [ABSTRACT FROM AUTHOR]

This corrects the article DOI: 10.1038/ncomms7761 [ABSTRACT FROM AUTHOR]